As an improvement over the spray painting of articles such as automobile bodies, boats and household appliances, a new kind of paint-coated sheet material (also called paint-coated film) has been developed to provide protective and decorative finishes. The new material comprises a flexible, stretchable, thermoplastic carrier film which has a protective and decorative paint layer of uniform thickness and appearance on one side, and, optionally, an adhesive layer on the other side. It can also have other layers such as a tie or bonding layer between the paint layer and the carrier film. In a preferred embodiment, a transparent topcoat covers the paint layer. This combination provides an attractive basecoat-clearcoat appearance which is especially desirable for automotive finishes.
Using known vacuum thermoforming procedures, the paint-coated film can be stretched and bonded to an article such as an automobile body panel to give a basecoat-clearcoat finish. Advantages over obtaining such finishes by the use of paint-coated film instead of by spray painting include improved control of evaporating solvents, which reduces air pollution, and the elimination or reduction of paint sludge as a hazardous waste product. Furthermore, the new paint-coated film has a remarkably smoother and more attractive appearance than spray painted finishes. This new type of thermoformable sheet material and a process for its manufacture are described in the U.S. patent application of G. G. Reafler, Ser. No. 116,426, filed Nov. 3, 1987. The process involves applying the paint composition to the surface of the thermoplastic carrier film by laminar flow coating, followed by drying and then coating and drying each additional layer in sequence to obtain a paint-coated film of excellent gloss and smoothness.
The paint-coated film can be stretched and bonded by vacuum thermoforming to a three dimensional substrate such as an automobile panel to form a bonded article. However, it has been found that as a result of the thermoforming procedure certain defects may develop in the resulting bonded article, especially if the article is subjected to high temperature over a period of time.
One such defect can result from the entrapment of air bubbles between the film and the substrate. When large enough, the bubbles are immediately visible after thermoforming as irregularities in the otherwise smooth paint-coated surface. Even small bubbles, however, can cause visible defects when the article is exposed to high temperature over a period of time, as occurs when an automobile is exposed to the sun in warm weather.
To avoid air entrapment when adhesively bonding a film to a substrate by vacuum thermoforming, others have thought it necessary to use porous substrates or to form holes in the substrate. In the patent to Thompson, U.S. Pat. No. 3,551,232, solid particles are added to the adhesive composition to facilitate air flow. These expedients, however, can cause visible patterns or defects in the surface of the adhesively bonded, thermoformed film.
Exposure to high temperature can also result in the defect known as "peel-back". This is believed to result from stress relaxation of the bonded film and is a consequence of stress created in the film when stretched by the thermoforming operation and bonded to a rigid substrate. When the bonded article is heated, for example, to 200.degree. F. for one hour, the adhesive bond weakens and, at least along the edges, the stretched film shrinks and peels back. A need exists, therefore, for a thermoforming method which will reduce or eliminate air entrapment and the risk of peel-back of the bonded film. The method should also produce a film surface which is smooth and glossy and free of the undesirable patterns which would be caused by holes or pores in the substrate or by particles in the adhesive composition. The present invention provides such a method by means of which a paint-coated film can be bonded to a non-porous, three-dimensional substrate with little or no air entrapment and with limited risk of peel-back at elevated temperatures.